The present invention relates generally to an image recording system for recording a color image. More specifically, the invention relates to an image recording system for correcting the deviation of position of a scanning line forming a recorded image, and for correcting the deviation of density of the recorded image based on the deviation of position of the scanning lines.
In addition, the present invention relates to a method for recording a pattern to detect the presence of deviation of color which is the deviation of relative position between pictures of the respective colors in a color image recording system, and a color-deviation detecting system for detecting the deviation of color using a pattern recorded by the method.
In image recording systems for outputting a color image signal prepared by a computer as a hard copy, so-called color printers, image forming processes of various types, such as an electrophotographic form, an ink-jet form and a thermal transfer form, have been made fit for practical use. In particular, in the present circumstance, the electrophotographic form is the most suitable for high picture-quality printers due to the advantage of the electrophotographic form being able to record an image of a high picture quality at a high speed. As is well known, in the electrophotographic form, a laser light having an intensity modulated in accordance with an image signal scans on a photosensitive drum to form an electrostatic latent image in accordance with the image signal, and the formed electrostatic latent image is developed by a toner to form a visible image.
In a color printer of the electrophotographic form, a color image is formed by performing the aforementioned series of image forming processes using toners of three colors including yellow, magenta and cyan, or using toners of four colors including black in addition thereto. The deviation of relative positions of scanning lines forming a recorded image between image forming systems of the plurality of colors, causes so-called color deviation, so that there is a problem in that the picture quality notably deteriorates. Specifically, color deviation appears in the form of bleeding produced on lines of red, green and so forth in the recorded image. Since the deviation of relative positions of scanning lines is visible even if its amount is about 150 .mu.m, this deviation must be less than this value in order to obtain a recorded image of a high picture quality.
Such a deviation of relative positions of scanning lines in an image forming system of a plurality of colors is caused by the distortion of a scanning line itself in image scanning systems of the respective colors, the deviation of relative scanning starting positions and so forth. The former is caused by the distortion of a light scanning system, the eccentricity of a photosensitive drum and so forth, and the deviation of color occurs if the characteristics of distortions of the respective colors are different. In addition, the latter is caused by the deviation of the mounting positions of developing machines for the respective colors and so forth. The distortion in the scanning line itself may correspond to a partial deviation from the original position of the scanning line if viewed locally. Therefore, throughout the specification, the distortion of the scanning line itself and the deviation of the relative scanning starting positions will be generally referred to as "the deviation of position of a scanning line".
As a method for preventing such a deviation of position of a scanning line, there are methods for fully increasing the scanning accuracy and the mounting accuracy of components. However, in these methods, there is a problem in that the manufacturing cost increases since it is required to precisely work parts and the assembling manday increases.
On the other hand, as a method for solving the problem of causing the color deviation at a low cost, there is known a method for correcting the deviation of positions of a scanning line by a signal processing (Japanese Patent Laid-Open Nos. 4-317247 and 4-326380). In this method, the deviation of position of a scanning line is previously stored, and the correction is performed by outputting the image data on the deviated position. Although this method has no advantageous effects with respect to random color deviation which can not predicted, it is an effective method for correcting the deviation of color due to the deviation of position of a scanning line since most of the distortions of scanning lines are put under the rule of components which can be predicted. In addition, there is known an example for applying the similar method to a color-deviation correction in a printer using solid luminescent elements such as a LED array, not a laser-light scanning system (Japanese Patent Laid-Open Nos. 4-291372 and 4-281476).
The principle of the aforementioned conventional method for correcting the deviation of position of a scanning line by signal processing will be described below in detail below.
In this method, the amount of deviation of a scanning line from its ideal position is predicted, and the correction is performed by outputting image data on the deviated position. For example, when a scanning line is distorted as shown by a solid line of FIG. 7, the image data on picture elements expressed by the sign .quadrature. are outputted to the scanning line, so that the image is recorded at a position which is to be originally recorded, regardless of the distortion of the scanning line. Furthermore, the prediction for the amount of the deviation of position of the scanning line is performed by previously measuring the amount of deviation or by measuring the amount of deviation before image recording.
However, in this method, since the positions of picture elements are shifted for correcting the deviation of position of the scanning line, the pattern of recorded picture-points varies. In particular, in the case of correcting the position of the scanning line in the vertical scanning direction, the correction can be only performed every unit of one picture element as the minimum unit, so that the correction of the position by one picture element may be performed even if the amount of deviation of the scanning line between the adjacent picture elements is considerably smaller than one picture element. For that reason, the correction causes the disappearance of the picture-point data and the variation of the relationship between the adjacent picture points. The disappearance of the picture-point data causes the deviation of density fluctuations (which will be hereinafter referred to as "density deviation") of the recorded image since the picture point to be originally present disappears. The variation of the relationship between the adjacent picture points also causes the density deviation due to the variation of the pixel pattern. In particular, in the electrophotographic recording, since the laser beam is usually wider than the size of a picture element, the state of the adjacent picture points varies so as to vary the pattern of picture points, so that the amount of toner fluctuates so as to fluctuate the density of the recorded image.
Referring to FIGS. 8A through 8C, specified examples will be described. The correction of deviation of position in the vertical scanning direction will be described herein. FIGS. 8A through 8C show positional-deviation signals in the vertical scanning direction, original image signals before the correction of positional deviation, and image signals after the correction of positional deviation, respectively. The number "0" in the positional-deviation signal represents that there is no deviation of position, and the number i (1, 2, . . . ) represents that the position of the scanning line deviates downwards by i picture elements in the figure. By this processing for correcting the deviation of position of the scanning line on the basis of the positional-deviation signal, the original image signals of FIG. 8B are corrected as shown in FIG. 8C.
As can be seen from the comparison between FIGS. 8B and 8B, the number of the picture points of the original image signals is doubled in the region expressed by P0 in the figure, the picture points disappear in the region expressed by P1, and the pattern of picture points varies in the region expressed by P2, so as to produce the deviation of density. Although these deviations in density are also observed in a binary image drawn by a line, they are particularly conspicuous in an image expressed by the pseudo tone processing such as the error diffusion method and the dither method.
In this way, according to the conventional method for correcting the deviation of position of a scanning line by signal processing, although the deviation of position itself can be decreased, there are problems in that the smooth line can not be drawn, the deviation of density in a pseudo tone image occurs, and a string-like noise is produced.
As mentioned above, in the conventional technique which corrects the deviation of position of a scanning line by signal processing in a color printer, there are problems in that a line can not be smoothly drawn in a recorded image, the density of an image expressed by the pseudo tone such as the dither method and the error diffusion method can not be correctly expressed, and a string-like noise is produced.
By the way, in a color printer having an individual recording stage for each color, if there are the deviation of relative recording position in the recording stage for each color, or if there is the deviation of a recording paper by a paper feeding mechanism, the deviation of relative position between the pictures by each color occurs. Such a deviation of relative position is generally called color deviation, which considerably deteriorates the picture quality of a recorded color image.
Therefore, there are proposed techniques for detecting the presence of color deviation to adjust the position of a recording stage and a paper feeding mechanism, and for detecting the amount of color deviation to move the coordinate point of the recorded image data by an amount corresponding thereto, to correct the color deviation. It is the point of the correction of color deviation how to the correction of the color deviation can be accurately and simply detected.
As a method for detecting the amount of color deviation, there is known a method for recording a detecting mark called "register mark" every color to observe the recorded register mark by a microscope, as is well known in the field of printing. However, it is very complicated that an user in general observes a register mark by a microscope.
On the other hand, Japanese Patent Laid-Open No. 6-118735 discloses a system for detecting a color deviation using an inexpensive detector. In this system, the amount of a color deviation is detected by recording angle color-deviation detecting marks which have different colors in printers for the respective colors, and by discriminating the time when the marks for the respective colors passing through a detector on the basis of the rise timing of an output signal of the detector. In this system, the detecting accuracy is considerably lowered when the detector is soiled by toner or paper powder or when the density of the color-deviation detecting mark varies. Moreover, since the detector is arranged within a printer, as the temperature inside of the printer increases, the rise characteristic of the output signal of the detector fluctuates to produce a detecting error.
As mentioned above, the method for detecting a color deviation by the observation by means of a microscope is very complicated. In addition, in the method for detecting a color deviation by detecting a detecting mark by a detector, a detecting error occurs due to the soiled detector, the variation of density of the mark, the fluctuation of the surrounding temperature and so forth, so that there are many problems with respect to its reliability.